Method of making pressure-sensitive resistor element

ABSTRACT

A method for producing a pressure-sensitive resistor element comprising the steps of mixing conductive particles with an elastic polymer resin, pulverizing said particles subsequent to heat-molding them to a semi-hardened state, effecting a renewed heat-molding operation of aggregated particles under pressure at a suitable temperature, and attaching electrodes to said heatmolded particles.

O United States Patent 1 1 1111 3,808,678 Kuhn et al. A May 7, 1974 METHOD OF MAKING 3,254,529 6/1966 Thurston... 338/2 x PRESSURESENSITIVE RESISTOR 3,382,574 5/1968 Chadwick 29/613 X 3,469,441 9/1969 Pohl 338/2 X ELEMENT 3,676,925 7/1972 Sato et a] 29/613 [75] Inventors; Shoichi Kubo; Masakazu Komatsu, 3,689,618 9/1972 Chadwick 264/104 both of Kashihara, Japan Primary ExaminerCharles W. Lanham [73] Asslgnee' i z g z z fg gs lndusma] Assistant Examiner-Victor A. DiPalma p Attorney, Agent, or Firm-Stevens, Davis, Miller & [22] Filed: Aug. 16, 1972 Mosher [21] Appl. No.: 281,028

[57] ABSTRACT I 52 US. Cl 29/610, 338/2, 338/5, A method for Producing a pressure-sensitive resistor 264/104 264/140 element comprising the steps of mixing conductive [51 1111. C1 H0lc 17/00 Particles with an elastic p y resin. p ri ing i [58] Field of Search 29/610, 613, 62]; 3338/2 particles subsequent to heat-molding them to a semi- 333 5 223 224 2125;2 1:4 40 104 hardened state, effecting a renewed heat-molding operation of aggregated particles under pressure at a [56] Reference Cit d suitable temperature, and attaching electrodes to said heat-molded particles. I 2,358,21 1 9/1944 Christensen et a1. 264/104 X 9 Claims, 7 Drawing Figures PATENTEBIAY 1 1914 SHKU 2 BF FIG. 2

RESISTANCE mm APPLIED LOAD (g) PATENTEBm 11814 3.808.678

' sum 3 or 4 FIG. 3

CURRENT (mA APPLIED LOAD (9) PATENTED 7 SHEET ls 0F 4 7 FIG. 7

*--- RESISTANCE (KS2 4'0 6 0 8 0 Ibo TENSVILE LOAD (g) METHOD OF MAKING PRESSURE-SENSITIVE RESISTOR ELEMENT The present invention relates to a pressure-sensitive resistor element and a method for producing the same.

Previously, pressure-sensitive resistor elements having elasticity, for converting mechanical stress into an electrical signal, have been made of conductive rubber prepared in such a manner that rubber is mixed with powders of natural or artificial graphite and then subjected to vulcanization molding to produce the conductive rubber.

Such conductive rubber, however, has disadvantageously resulted in small variation rate of resistance relative to the mechanical stress and in large hysteresis. The conductive rubber, for example, mixed with 20% by weight of graphite, has only a variation rate of resistance as small as 1.5 even when the mechanical stress as much as 4 kg/cm is applied thereto, the variation rate of resistance being here termed as .the ratio of the resistance value with no stress to the resistance value with a given stress.

Further, it was also difficult to attain uniform distribution of conductive particles among the rubber, thus resulting in an unstable pressure-sensitive resistor element.

An object of the present invention is to provide a pressure-sensitive resistor element having a large variation rate of resistance with respect to a given mechanical stress applied thereto and having small hysteresis.

Another object ofthe invention is to provide a In the drawings:

FIG. 1 is a perspective view of a pressure-sensitive resistor element prepared according to the present invention;

FIG. 2 is a characteristic curve of resistance to applied load in a pressure-sensitive resistor element prepared according to the present invention;

FlG. 3 is a hysteresis curve of current against load with a given voltage applied across a pressure-sensitive resistor element prepared according to the present invention;

FIG. 4 is a cross-sectional view of a pressure-sensitive resistor element prepared according to the present invention wherein said element is covered with insulating tapes;

FIG. 5 is a cross-sectional view of a pressure-sensitive resistor element prepared according to the present invention wherein said element is covered with an elastic resin;

FlG. 6 is a perspective view of a pressure-sensitive resistor element prepared according to the present invention wherein said element is mounted on an elastic plate flexible responsive to tensile load; and

FIG. 7 is a diagram of a characteristic curve of resistance in terms of tensile load as to a pressure-sensitive resistor element as shown in FIG. 6.

The present invention has objects to provide a pressure-sensitive resistor element having high sensitivity and good stability and to provide a method for producing the same wherein conductive particles are coated with a hardened elastic polymer resin and then subjected to heat-molding. In one embodiment of the present method, the conductive particles are kneaded into a mixture with the elastic polymer resin and then heated to a semi-hardened state and conductive particles coated with the polymer resin are prepared by pulverizing the semi-hardened particles. The thus coated conductive particles are aggregated and molded by applying renewed heat treatment to thereby fabricate a pressure-sensitive resistor having small hysteresis with a uniform structure and having a large variation rate of resistance relative to the applied mechanical stress.

Then, metallic electrodes are attached thereto by the use of a method of vapour-evaporation or by means of adhesive agents in order to provide a pressure-sensitive resistor element.

The application of the mechanical stress to the thus fabricated pressure-sensitive resistor element causes a contact surface between the conductive particles and contact pressure therebetween to be varied to thereby change the value of resistance.

Further, a smooth elastic deformation can be seen due to elasticity of polymer resin serving as a binder among the conductive particles.

The conductive particles, for example, are such particles as carbonaceous powders of to 600 mesh prepared by carbonizing or graphitizing a 'thermosetting resin selected from phenol or furan resins. The spheric particles, generally designated as glass-like carbon, are particularly suitable for conductive particles as compared with normal natural graphite or artificial graphite due to the facts of (l) the large variation rate of contact resistance; (2) strong duration against heat impact such as a spark or are discharge; (3) high mechanical strength; (4) isotropic nature relative to electric resistance; and (5) conformability to a spheric form. Thus the glass-like carbon is adapted for use as the conductive particles from which a pressure-sensitive resistor having a large variation rate of resistance and good stability is fabricated.

On the other hand, silicon resin having resistance to heat and chemical materials is preferably employed as the polymer resin.

The mixture ratio of the conductive particles to the polymer resin may be selected to be by weight 3/7 to 7/3 from the viewpoints of variation rate of resistance and stability.

The adhesion of the electrodes to the resistor is of great importance, and a good result can be obtained when the electrodes are adhesively fixed to the elastic pressure-sensitive resistor with the aid of a conductive adhesive agent. The electrodes have conventionally been attached thereto with a thermosetting conductive adhesive, or by a metallic vapor-evaporation or metallic coating method.

However, problems arise in such an adhesion; for example, there occurs a crack or exfoliation on the electrodes immediately upon the repeated application of stress because of the great difference of Young modulus of the electrode from that of the adhesive due to the elasticity of the pressure-sensitive resistor, thus resulting in the occurrence of unstable contact resistance between the elastic pressure-sensitive resistor and the electrode, thereby providing an unstable pressuresensitive resistor element.

On the other hand, a conductive pressure-sensitive adhesive agent wherein the pressure-sensitive adhesive is kneaded in mixture with conductive powders such as silver powder exhibits a stable characteristic and prevents the electrode from being exfoliated from the pressure-sensitive resistor because it'has the same elasticity as the pressure-sensitive resistor.

In FIG. 1 there is shown a structure of the abovementioned pressure-sensitive resistor element wherein a pressure-sensitive resistor layer 3 is sandwiched by metallic electrodes 1 and 2, which are, respectively, fixed to the corresponding pressure-sensitive resistor layer with conductive pressure-sensitive adhesive agents 5 and 6. The pressure-sensitive resistor element has the value of its resistance changed when the stress is applied thereto into the direction of an arrow A.

FIG. 2 shows a resistance-load diagram of the pressure-sensitive resistor element according to the present invention.

In this embodiment, the glass-like carbonaceous particles having 250 to 300 mesh, made of a graphitized' and carbonized thermosetting resin, are mixed with a weight ratio of 50 to 50 and then heated to obtain the semi-hardened particles. By pulverizing the so produced particles, the carbonaceous particles coated with the polymer resin are prepared, and they are again aggregated so that molding operations can be carried out under pressure at a suitable temperature to obtain the pressure-sensitive resistor. Then, the complete pressure-sensitive resistor element can be fabricated by providing the pressure-sensitive resistor with electrodes made of copper on both surfaces thereof by the use of the'conductive pressure-sensitive adhesive prepared by mixing a pressure sensitive adhesive made of silicon with silver powder.

A loading test was conducted as to the sample of the thus prepared pressure-sensitive resistor element hav-. ing a dimension of an outer diameter of 5 mm and a thickness of 0.8 mm with the copper electrodes attached thereto. The result is illustrated in FIG. 2 and Table I, revealing a large variation rate of resistance,

i.e. 1.2 X

I TABLE I APPLIED LOAD RESISTANCE o g 120 M0 800 g l0 0 rate of resistance the value of which rangeswithin 5% with respect to the initial value with an improved high resistance variation rate as compared with the conventional conductive rubber, thus confirming the provision of a high stability pressure-sensitive resistor element. Preferably,.the above-mentioned pressure-sensitive element may be covered with an insulating envelope for mechanical protection because of' its insufficient mechanical strength; alternatively, it may be covered with the elastic polymer resin along the circumference thereof with a view of increasing the moistureresistance or mechanical strength.

In FIG. 4 there is shown another embodiment of the pressure-sensitive element wherein the element 7 is covered and printed by means of epoxy-adhesive tapes 8 and 9 encapsulated in a glass tube with leads l0 and 11 attached to the electrodes.

On the other hand, FIG. 5 shows still another embodiment of the pressure-sensitive, resistor element in which a pressure-sensitive resistor 12 is provided with electrodes 13 and 14 and covered with a silicon resin 15 therearound. I

The pressure-sensitive resistor element prepared according to the present invention tends to be brittle when the tensile load is applied thereto. In order to remove the drawback, the pressure-sensitive resistor .16 provided with electrodes 17 and 18 on both sides thereof is adhesively fixed to flexible elastics 19 with an adhesive as shown in FIG. '6. As a result, the application of thetensile stress to the elastic plate 19 in the BB, direction allows the indirect application of the tensile load to the pressure-sensitive resistor 16. Thus, this method makes possible the protection for a mechanically fragile pressure-sensitive resistor, further providing the possibility of arbitrarily changing the tensile load-resistance characteristic by selecting the material of the elastic plate.

FIG. 7 shows the resistance-tensile load characteristic of a pressure-sensitive resistor element having the structure as shown in FIG. 6.

In this embodiment, cabonaceous particles having 250 to 300 mesh prepared by carbonizing and graphitizing a thermosetting resin is mixed with a silicon resin with a weight ratio of 60 to 40 and then heated to a semi-hardened state to prepare the carbonaceous particles covered with the resin through a step of pulverizing the semi-hardenedparticles. After that, these particles are again aggregated and subjected to molding operations under pressure at a suitable temperature to obtain a pressure-sensitive resistor having a dimension of an outer diameter of 5 mm and a thickness of 0.8 mm. As shown in FIG. 6, the pressure-sensitive resistor is then provided with electrodes 17 and 18 on both ends thereof with a conductive pressure-sensitive adhesive, and is adhesively fixed to an elastic plate 19 made of natural rubber 0.5 mm thick to accomplish the complete pressure-sensitive resistor element. The thus prepared element exhibits a highly stable resistance-load characteristic as shown in FIG. 7 when the tensile load is applied thereto.

What is claimed is: l. A method of making a pressure-sensitive resistance element, comprising the steps of:

l. mixing particles of an electrically conductive material and an elastic polymer resin; 2. heating the mixture of step (I) to a semi-hardened state; 3. pulverizing the semi-hardened mixture of step (2) to obtain particles coated with said polymer resin;

4. heating the'pulverized particles of step (3) under pressure to heat-mold said pulverized particles and form an aggregation of a desired shape; and

5. attaching electrodes to said aggregation of desired shape to form said pressure-sensitive resistance element.

2. The method according to claim 1, wherein said electrodes are attached to opposite surfaces of said aggregation with a conductive pressure-sensitive adhesive material.

3. The method according to claim 2, wherein said adhesive material is prepared by mixing silicon with silver powder.

4. The method according to claim 1, wherein said conductive particles are carbonaceous powders of between about 100 and 600 mesh prepared by carbonizing and graphitizing a therrnosetting resin selected from the group consisting of phenol and furan resins.

5. The method according to claim 4, wherein said polymer resin comprises a heat resistant silicon resin.

6. The method according to claim 5, wherein the mixture ratio by weight of conductive particles and polymer resin is selected from the range of between about 3:7 and 7:3.

7. The method according to claim 4, wherein said conductive particles have a mesh of between about 250 I and 300.

8. The method according to claim 7, wherein said conductive particles and resin particles are mixed with a weight ratio of approximately 111.

9. The method according to claim 7, wherein said conductive particles and resin particles are mixed with a weight ratio of approximately 3:2. 

2. The method according to claim 1, wherein said electrodes are attached to opposite surfaces of said aggregation with a conductive pressure-sensitive adhesive material.
 2. heating the mixture of step (1) to a semi-hardened state;
 3. pulverizing the semi-hardened mixture of step (2) to obtain particles coated with said polymer resin;
 3. The method according to claim 2, wherein said adhesive material is prepared by mixing silicon with silver powder.
 4. The method according to claim 1, wherein said conductive particles are carbonaceous powders of between about 100 and 600 mesh prepared by carbonizing and graphitizing a thermosetting resin selected from the group consisting of phenol and furan resins.
 4. heating the pulverized particles of step (3) under pressure to heat-mold said pulverized particles and form an aggregation of a desired shape; and
 5. attaching electrodes to said aggregation of desired shape to form said pressure-sensitive resistance element.
 5. The method according to claim 4, wherein said polymer resin comprises a heat resistant silicon resin.
 6. The method according to claim 5, wherein the mixture ratio by weight of conductive particles and polymer resin is selected from the range of between about 3:7 and 7:3.
 7. The method according to claim 4, wherein said conductive particles have a mesh of between about 250 and
 300. 8. The method according to claim 7, wherein said conductive particles and resin particles are mixed with a weight ratio of approximately 1:1.
 9. The method according to claim 7, wherein said conductive particles and resin particles are mixed with a weight ratio of approximately 3:2. 